Lo-Dash sortBy array of dates in string format

Question

I'd like to know why lodash doesn't sort array of dates in string format as compared with plain javascript sort(). Is it expected behavior or a bug?

var array = ["2014-11-11", "2014-11-12", null, "2014-11-01", null, null, "2014-11-05"];

_.sortBy(array);
// ["2014-11-11", "2014-11-12", null, "2014-11-01", null, null, "2014-11-05"]

_.sortBy(array, function(value) {return new Date(value);});
// [null, null, null, "2014-11-01", "2014-11-05", "2014-11-11", "2014-11-12"]

array.sort();
// ["2014-11-01", "2014-11-05", "2014-11-11", "2014-11-12", null, null, null]

Version used: Lo-Dash v2.4.1 – Modern build.

Kiril · Accepted Answer · 2014-11-14 17:25:18Z

28

If you take a look to lodash code you may see how it's implemented. Function _.sortBy inside uses native Array.prototype.sort (see source). But the root is not there. More interesting is function compareAscending that is passed as a callback to native sort (source). So in a few words your

_.sortBy(array, function(value) {return new Date(value);});

is converted to:

array.sort(function(a, b) {
    var aa = new Date(a),
        bb = new Date(b);

    if (aa !== bb) {
        if (aa > bb) { return 1; }
        if (aa < bb) { return -1; }
    }
    return aa - bb;
})

So why nulls are in the beginning? Because new Date(null) returns Thu Jan 01 1970 01:00:00 which is less than any other date in your array.

What about native sort? According to spec (see here) The default sort order is according to string Unicode code points. If simply - native sort converts items to strings and compares strings. So native sort is smth like:

_.sortBy(array, function(value) {return value + ''; });

As soon as 'null' string is always "bigger" than date string (like '2014-11-11') - nulls will be in the tail of the result array.

answered Nov 14, 2014 at 17:25

Kiril

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1 Comment

yerforkferchips Over a year ago

You can also use _.sortBy(array, String), which uses the fact that String, as a function, also coerces a given value to a String, just like +''.

Darsshan · Accepted Answer · 2019-11-21 12:24:51Z

3

If you are trying to use lodash to sort dates in ascending or descending order for your array of objects, you should use _.orderBy instead of _.sortBy

https://lodash.com/docs/4.17.15#orderBy, this method allows specifying sort orders either by 'asc' or'desc'

An example would be:

const sortedArray = _(myArray.orderBy([
  function(object) {
    return new Date(object.date);
  }],["desc"])

answered Nov 21, 2019 at 12:24

Darsshan

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